Disclosed are concepts related to rockets used to launch payload such as satellites into orbit. Re-usable rockets would dramatically reduce the cost of launch services. Starting a launch while at higher altitudes enhances the performance of the rocket engines, and if traveling at airplane cruising speed, the cruising speed is added to the rocket's final velocity, improving its payload size and weight capabilities or increasing its final speed and altitude. Various methods have been proposed to assist a rocket to allow it to start powered flight at altitude.
Satellites have become a necessary part of life in the last 50 years. We use GPS to navigate our cars, airplanes and ships. We use satellites for internet, radio, TV and cell phones. Government satellites monitor the weather, climate and activities on Earth. Google Earth allows us to see the entire planet from satellite views, with higher resolution each year. Also payloads such as supplies to the International Space Station are necessary on a regular basis. To keep so many satellites in orbit, and to constantly add new ones, many rockets are launched each year. It would be less expensive and easier to operate if rockets can launch from Earth, fly to orbit, deliver a payload to orbit, return to Earth and be ready to launch again the next day. Such a rocket would be classed as a reusable single stage to orbit rocket. Some patents have been issued for single stage to orbit designs, such as U.S. Pat. Nos. 5,667,167 and 5,842,665. However at present no single stage to orbit vehicles have been successfully built and flown to orbit. The problem is the amount of fuel and oxidant required is approximately 97-98% of the total gross weight at launch. This leaves only 2-3% for the vehicle, the payload, and the engines.
Instead of single stage to orbit, multiple stages are used, with the first and second stages usually being expendable—i.e. they are tossed away after their fuel has been exhausted. Often a third stage is also expendable. The final part that gets to orbit is small, but better than 2-3%. Some companies are working on reusable early stages, but at present, most are used once and not able to be used a second time. Reusable stages reduce the cost and increase the safety. Being able to test a vehicle before its commercial flights allows the engineers to find any problems and correct them before the vehicle is exposed to its maximum stress.
Many companies have designed first stages to launch the rocket assembly, and then fly back to Earth. Usually they have wings and the winged first stage glides back to a landing strip. Some designs include extra air-breathing jet engines for the return flight, or sufficient fuel and oxidant and a rocket engine that can be run at lower thrust and can be used to power the return flight. This allows the first stage to be safely returned and reused afterwards. U.S. Pat. No. 6,612,522 proposes a reusable first stage booster that can fly back to the launch pad. Kistler Space Systems' U.S. Pat. Nos. 6,158,693 and 5,927,653 propose a parachute system for the safe return. Others such as SpaceEx and Blue Origins have designed first stage rockets that return and use their rocket engines to land vertically. Some have returned safely, but at present most vertical landing systems have failed to land properly and it remains difficult to have a first stage return this way. Airplane style vehicles are well proven and understood technology, and return of winged vehicles under power or gliding are common and generally very safe. If the first stage is essentially an airplane, then the return is a standard airplane return to the runway and landing.
If the first stage vehicle takes off vertically as described in U.S. Pat. No. 6,612,522, and then returns and lands on an airport runway, the setup and preparation of the launch require all of the complex launch pad and services of a standard vertical ascending rocket. It reduces the required structure of the first stage compared to a runway takeoff, but it requires a launch pad and support structure. Usually the infrastructure to provide secure support before launch requires a system such as used for launches at the Kennedy Spaceport—which cost billions of dollars to build. If instead a first stage vehicle can take off from a standard airport runway, then the infrastructure is already available at many airports and the cost per flight is minimal. However, having an airplane-like vehicle for the first stage, or rocketplane, that can takeoff from a runway has some challenges. The rocketplane requires large amounts of weight for the fuel and oxidant in the vehicle at takeoff. To handle the takeoff weight, the wings need to be large enough to provide sufficient lift at low speeds, and the landing gear needs to be able to handle the weight. The entire structure of the vehicle needs to be stronger than if the fuel and oxidant was of minimal weight. At present, there are designs in process for such a rocket vehicle to act as a first stage, such as Xcor Aerospace's Lynx rocketplane, and U.S. patent application Ser. No. 11/408,164 but none has been actually built and are flying at present.
In all cases, for an airplane to take off from the ground, it must have more lift than total weight. This total weight must equal the weight of the engines, the vehicle itself, the fuel and the cargo or payload. For any given design of vehicle the amount of fuel and the weight of the cargo can be adjusted to fit the vehicle's ability to lift off. For instance, if traveling a short distance, it is regular practice to not fill the fuel tanks so that one can add more cargo. If traveling the longest distance for the airplane's fuel capacity, then the cargo is restricted to a lower maximum.
The lift of wings increases as the air speed increases. Airplanes can take off on a shorter runway if the airplane is not fully loaded to its maximum lift capability. The military often use short runways such as on an aircraft carrier by starting the airplane with minimal fuel in its tanks and then transferring fuel to the airplane while in flight. This also increases the range of the airplane by replenishing the fuel spent to attain altitude.
It would be desirable to have the first stage rocket powered airplane (rocketplane) be able to take off with little fuel and oxidant in the vehicle, and load the fuel and oxidant while in flight. This would enable the minimization of the vehicle structure, landing gear, and wing size. Such a reduction could save approximately 20% of the takeoff weight. As noted above, the payload could only be 2-3% of the total vehicle weight if the takeoff starts at sea level with all fuel and oxidant in the vehicle. Adding the fuel and oxidant in flight would allow the payload and booster to be up to 20% of the total vehicle weight, allowing the payload to be significantly heavier than would be possible otherwise.
Several designs have tried to enable this concept by carrying the rocketplane on a larger airplane. For instance, Stratolaunch is designed to carry a fully loaded rocket or rocketplane to high altitude, of approximately 40,000 feet, and release the rocket or rocketplane at said altitude, and at a speed in the range of 500-1000 km/hour (300-600 mph) which is the normal cruising speed at 40,000 feet for a large jet airplane. This accomplishes the desired result of enabling the rocketplane to begin its flight at high altitude and with significant initial speed, but requires a very expensive dedicated vehicle to carry the rocket or rocketplane to altitude, and requires a complex and expensive procedure to connect the two vehicles before flight. Also, the rocketplane still requires significant structural strength since the vehicle is still carrying the full fuel and oxidant load at the start of the flight.
U.S. Pat. Nos. 5,295,642, 5,456,424, 5,564,648 and 6,119,985 have proposed transferring fuel and oxidant in flight. A larger tanker airplane carries the fuel and oxidant, and the rocketplane flies to altitude with its own power using jet engines or rocket engines, connects with the tanker's fuel and oxidant lines, and loads the fuel and oxidant in flight. When the tanks are full, it separates from the tanker and uses its rocket engines to fly to a very high altitude where a payload and booster rocket are released to go to orbit. The rocketplane glides back to regular flight altitudes and then either glides back to an airport or uses its engines to fly back to an airport. This is accepted as a practical method to accomplish a low weight takeoff and high capacity of fuel and oxidant for the rocket powered portion of the flight. The challenge has been to prove the safety of the in-flight transfer of fuel and oxidant. The transfer of fuel in flight is a daily event with the air force, and is accepted as safe. However, the transfer of oxidant at the same time has not been demonstrated, and there are concerns about spillage of the oxidant getting into the jet engines and damaging them, or the spillage mixing with any fuel spillage and igniting, and of the difficulties of transferring cryogenic oxidant through standard hoses and connectors similar to those presently used for jet fuel.
The second problem of in-flight transfer of fuel and oxidant is that the rocketplane receiving the fuel and oxidant gains in weight as it flies. With the engines running at full power to maintain cruising speed, the added weight can only be compensated for by both the tanker and rocketplanes dropping in altitude during the transfer—estimated to be a drop from 40,000 feet to as low as 25,000 feet. This removes a significant part of the benefit of starting the rocket engines in rarefied atmosphere, which would allow the engines to be better tuned to minimal atmosphere which makes the engines more efficient.
U.S. patent application Ser. No. 14/708,197 discusses a design in which fuel and/or oxidant is transferred from an attached external tank to the rocket to ensure that the tanks are toped to full status before launch. But the rocket and its universal carry support system are designed to be independent of the carrier airplane and able to be attached to many models of airplanes. As such, the external tank is attached to the support system, which effectively is part of the rocket and is an expendable part of the rocket system, not part of the carrier airplane. It commences the flight with the rocket fully loaded and only uses the expendable external tank to top off the tanks. It does not allow the rocket to be carried with minimal fuel and/or oxidant.
U.S. Pat. Nos. 5,626,310 and 6,029,928 propose the concept of towing a rocketplane to a cruising altitude of 30,000-40,000 feet, with full fuel and oxidant tanks on takeoff. At cruising altitude the towing aircraft would release the towed rocketplane, and the towed rocketplane would start its rocket engines, and fly to very high altitude where it would release its payload and booster.
The problem with the tow concept is the potential danger on takeoff of an aborted takeoff. If the towing airplane needs to abort the takeoff, the towed rocketplane is directly behind it. The towed rocketplane is essentially a bomb, with both large amounts of fuel and oxidant on board. If the towed rocketplane should impact the towing airplane the potential for a devastating explosion is high. While aborts on takeoff are rare, the potential exists.
Another problem is that the towed rocketplane requires enough strength in the structure and landing gear to support the full weight of the fuel and oxidant. This minimizes the benefits of assisted launch from the towing airplane. In-flight fueling allows for a lighter structure since the takeoff weight does not include most of the fuel and oxidant. Kelly proposed having detachable wheels and supports for the takeoff that remained on the runway, but the vehicle structure still must be stronger, and therefore heavier, than a vehicle taking off with minimal fuel and oxidant.